Communicationsto the Editor References a n d Notes (1) G. Blyholder, "Modern Aspects of Electrochemistry", Vol. 8, J. O'M. Bockris and B. E. Conway, Ed., Plenum Press, New York, N.Y., 1972, Chapter 1. (2)W. H. Weinberg, J. Vac. Sci. Tech., I O , 89(1973). (3)J. R. Schrieffer, J. Vac. Sci. Tech., 9, 561 (1972). (4)T. B. Grimley, "Molecular Processes on Solid Surfaces", E. Drauglis, et ai., Ed., McGraw-Hill, New York, N.Y., 1969,p 299. (5) T. B. Grimley, J. Vac. Sci. Tech., 8, 31 (1971). (6)G. Blyholder, J. Phys. Chem., 68, 2772 (1964). (7)G. Doyen and G. Ertl, Surface Sci., 43, 197 (1974). (8) J. C.Robertson and C. W. Wilmsen, J. Vac. Sci. Tech., 9, 901 (1972). (9)P. Politzer and S. D. Kasten, Surface Sci., 36, 186 (1973). (IO) A. J. Bennett, B. McCarroll. and R. P. Messmer, Phys. Rev. B, 3, 1397
(1971). (11) D. J. M. Fassaert, H. Verbeek, and A. Van der Avoird, Surface Sci., 29,
501 (1972). (12)J. A. Pople and D. L. Beveridge, "Approximate Molecular Orbital Theory", McGraw-Hill, New York, N.Y., 1970.
761
(13)G.Blyholder, Surface Sci., 42, 249 (1974), (14)0.Blyholder, Chem. Commun., 625 (1973). (15)G.Blyholder, J. Vac. Sci. Tech., accepted for publication. (16)C. C. J. Roothaan, Rev. Mod. fhys., 23, 69 (1951). (17)J. A. Pople, 0.P. Santry, and G. A. Segal. J. Chem. Phys., 43, SI29 (1965). (18) J. A. PoDle and G. A. Seaal, J. Chem. Phys.. 43, S136 (1965);44, 3289 (1966). D. P. Santry and G. A. Segal, J. Chem. fhys., 47, 158 (1967). R. C. Baetzold, J. Chem. Phys., 55, 4335 (1971). D. W. Clack, N. S. Hush, and J. R. Yandle, J. Chem. Phys., 57, 3503 (1972). G. Blyholder, accepted by J. Res, Inst. Catab, Hokkaidb Univ. L. 0. Brockway and P. C. Cross, J. Chem. Phys., 3, 828 (1935). J. C. Tracy, J. Chem. fhys., 56,2736(1972). R . R. Ford, Adv. Catai., 21, 51 (1970). R. L. Park and H. E. Farnsworth, J. Chem. Phys., 43, 2351 (1965). I. H. Hillier and V. R. Saundero, Mol. fhys., 22, 1025 (1971). R. V. Culver and F. C. Tompkins, Adv. Catab, 11, 67 (1959). G. Blyholder and M. Allen, J. Am. Chem. Soc., 91, 3158 (1969). L. H. Jones, J. Chem. Phys., 28, 1215 (1958).
COMMUNICATIONS TO THE EDITOR
An Infrared Study of Some Reactions with Reactive Sites on Dehydroxylated Silica Publication costs assisted by imperial Oil Limited
Sir: A previous infrared spectroscopic study of the reactions of BC13 and of BF3 on silica1B2indicated that a new reactive site was formed as the isolated SiOH groups were removed by thermal degassing under vacuum. This new site was associated with the growth of bands in the background spectrum of silica a t 908 and 888 cm-' (see ref 2) as the intensity of the 3748-cm-l SiOH band decreased in intensity and was presumed to be a strained siloxane bridge although proof was lacking. In the present communication, a study of the chemisorption of H20, "3, and of CH3OH on highly dehydroxylated silica has provided further evidence that this site corresponds to a siloxane type site which forms when the degassing temperature under vacuum is greater than 400'. In addition a new type of SiOH species is formed following the reaction of the aforementioned molecules with these sites. Details of silica sample disk preparation3 (10 mg of SiOz/ cm2) and of the reaction cell4 have been described elsewhere. All adsorption reactions were carried out at room temperature. Following admission of 10 Torr of H2O to a silica sample which had been degassed a t 1100' under vacuum the bands at 9081'888 cm-l immediately disappeared and a new broad band a t 3741 cm-l appeared as a shoulder on the side of the normal isolated SiOH band at 3748 cm-' (Figure 1A). When the sample was titrated with small incremental doses of H2O (1-5 wmol) the 908/888 cm-l bands disappeared in proportion to the growth in intensity of the 3741-cm-l
band but the intensity of the band a t 3748 cm-l did not change. When a degassed deuterated silica (normal SiOD band at 2764 cm-l) was similarly titrated with H2O only the broad 3741-cm'l band was observed (Figure IB) and its halfwidth was about 19 cm'l. When DzO was added to a hydrogen containing silica, the corresponding broad band appeared at 2758 cm-'. Upon adding H2l8O (98.6% oxygen18) a very broad ( ~ 1 1 2= 30 cm-l) band appeared a t 3735 cm" (Figure 1C) with shoulders to low and high wave number. This band could be reconstructed from the sum of two identical broad bands with half-widths of 19 cm-l centered a t 3741 and 3730 cm-l, one corresponding to a Si160H and the other to a Sil*OH (Figure 1D). [The l80 isotope shift2 for the 3748-cm-I SiOH band is 11 cm-l.] Using 3" for the titration, a somewhat more symmetrical and narrower ( ~ 1 1 2= 12 cm-l) band appeared a t 3741 cm-' (Figure 1E) as the 908/888 cm-l bands disappeared and these changes were accompanied by the parallel growth of infrared bands a t 3540, 3450, and 1550 cm-l due the SiNH2 group^.^ An identical band a t 3741 cm-l also appeared when CH30H was used and bands due to SiOCH3 were detected in the vCH region.6 The 908/888-cm-l bands were just detectable if the degassing temperature was 400', as was the broad 3741-cm-1 band if H20 or NH3 was added. With higher degassing temperatures, all bands increased in intensity indicating the formation of an increasing number of reactive sites with increasing degassing temperatures up to a maximum of about 1200' (the softening point of quartz) when the silica was totally dehydroxylated. Prolonged evacuation a t room temperature or up to about 300' did not result in any alteration of the intensity The Journal of Physical Chemistry. Vol. 79. No. 7. 7975
762
Communications to the Editor 50r
be expected with equal intensities and separated by 11 cm-’. It is difficult to account for the results on the assumption that the surface is oxygen deficient and contains Si radical sites (further, no ESR signals have been detected for degassed samples2) or are
-si/p si0 ’
3770
3750
3730
3710
CM-’ Figure 1. (A) Dashed line represents the residual 3748-cm-I SiOH band on a silica which had been degassed under vacuum at about 1100’. Solid line spectrum was obtained after admitting 10 Torr of H20 at 20’ and evacuating the excess. (B) After addin 5 pmol of H20 to a degassed deuterated silica. (C) After adding H2Q80 to a degassed deuterated silica. (D) Taking two spectra as in B, displacing one by 11 cm-’ to 3730 cm-’, and summing the two (see text). (E) After adding 5 Mmol of NH3 to a degassed deuterated silica and evacuating the excess. The % Tscale refers to Figure A.
of the 3741-cm-1 band (or of the bands due to SiNHz when present) but with increasing temperatures (>400’) the 3741-cm-’ band diminished in intensity (as did the SiNHz bands) and the 908/888-cm-l bands reappeared. At no stage in this work were any bands observed between 2400 and 2000 cm-l which could be attributed to the formation of SiH containing species. Further, no reaction took place between degassed silica and CzH4, HCN, HCI, 0 2 , Hz, or PH3. Assuming that the new site corresponds to a “reactive” siloxane bridge site, the stoichiometry for reaction could be represented as follows:
>
SiOH
/y\ ’/I\ 0 ‘Si
HZO
+ NH,
CH,OH
-+
‘SOH
lSiNH2
>
7
/
SiOCH,
whereas for the boron compounds previously studied1,Z (BFa, BC13, BzHc), the reaction is: 0
’‘Si
+ BX,-
\ -SSiOBX, /
\ + -Six
/
For reaction with H2180 a pair of broad SiOH bands are to The Journal of Physical Chemistry. Vo! 79. No. 7. 7975
type sites involved as has been postulated by Low from the pyrolysis of methylated silica.7 We have also considered the possibility that geminal functional groups are formed and that the somewhat broader band at 3741 cm-l when HzO is the reactant is due to coupling between the identical Si(OH)z groups. However, we have rejected this since the peak position and band width remained constant for any H/D ratio (e.g. and 1:9 HzO-DzO mixture should give a spectrum in the uOH region which contains 95% SiOHSiOD pairs and only 5% SOH-SiOH pairs). Nonetheless, the unusual breadth and slight low wave number shift of the 3741-cm-’ band relative to the 3748-cm-’ SiOH band might reflect a very weak type of hydrogen bond interaction with the neighboring S O H , SiNHz, or SiOCHs groups, or it might simply be a reflection of some unusual geometry. The existence of reactive siloxane bridge sites which are formed as a result of the thermal dehydroxylation of silica has been suggested by Such sites might account for many of the anomalies (Le., high surface chlorine values) which have been noted in studies of the chlorination of degassed silica by hydrogen sequestering agents.12,13 We also note that the 400’ pretreatment for the initial appearance of these sites coincides with maximum temperature to which silica gels can be heated before rehydration becomes somewhat irreversible and the onset of hydrophobicity starts.14 Further details of our investigations of the nature of these sites will be reported later.
Acknowledgment. Support for this work was provided by Imperial Oil Ltd. and by the National Research Council of Canada. References and Notes (1) B. A. Morrow and A. Devi, Chem. Commun., 1237 (1971). (2) B. A. Morrow and A. Devi, J. Chem. SOC., Faraday Trans. 1, 68, 403 (1972). (3) B. A. Morrow and I. A. Cody, J. Phys. Chem., 77, 1465 (1973). (4) 6. A. Morrow and P. Ramamurthy, J. Phys. Chem., 77, 3052 (1973). (5) G. A. Blomfield and L. H. Little, Can. J. Chem., 51, 1771 (1973). (6) B. A. Morrow, J. Chem. SOC.,Faraday Trans. I , 70, 1527 (1974). (7) M. J. D. Low, J. Catal., 32, 103 (1974). (8) J. Kunavicz, P. Jones, and J. A. Hockey, Trans. Faraday Soc., 67, 848 (1971). (9) R. J. Pegiar, F. H. Hampleton, and J. A. Hockey, J. Catal., 20, 309 (1971). (10) E. Borello. A . Zecchine, and C. Morterra. J. Phys. Chem., 71, 2938, 2945 (1967). (11) C. Clark-Monks and 8. Ellis, J. Colloid lnterface Sci., 44, 37 (1973). (12) M. L. Hair and W. Hertl, J. Phys. Chem., 73, 2372 (1969). (13) M. L. Hair and W. Hertl, J. Phys. Chem., 77, 2070 (1973). (14) G. J. Young, J. Colloidlnterface Sci., 13, 67 (1958).
Department of Chemistry University of Ottawa Ottawa, Canada K7N 6N5 Received January 6, 1975
B. A. Morrow’ 1. A. Cody
